1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a demodulation method and system for a low-power differential sensing capacitive touch panel.
2. Description of Related Art
The modern consumer electronics are typically provided with touch panels for use as an input device. To meet with the requirements of light and compactness, a touch pad is usually combined with a display panel to form a touch panel for allowing a user to conveniently input data. Upon the sensing principle, touch pads are classified into four types including resistive type, capacitive type, surface acoustic wave type, or optics type.
The principle of a touch panel is based on different sensing manners to detect a voltage, current, acoustic wave, or infrared to thereby detect the coordinate of a touching point on a screen as touched by a finger or other object. For example, a resistive touch panel uses a voltage difference between the upper and lower electrodes to compute the position of a pressed point for detecting the location of the touching point, and a capacitive touch panel uses a capacitance change generated in electrostatic combination between the arranged transparent electrodes and the touching part of a human body to generate a current or voltage for detecting the coordinate of the touching point.
FIG. 1 is a schematic view of driving a typical n×m capacitive touch panel, where n, m are each an integer greater than one. As shown in FIG. 1, a signal generator 110 sequentially generates a driving signal Vin at each of the conductor lines X1 to Xn in one direction, and mutual capacitances C(1,1) to C(n,m) produced between the conductor lines X1 to Xn and the conductor lines Y1 to Ym in the other direction are employed to couple charges into the conductor lines Y1 to Ym. The detection circuit 120 has m sensing circuits 121 for measuring the charges so as to generate corresponding voltage signals Vo_1 to Vo_m.
When there is no grounded conductor or finger is close to the touch panel, the value of the mutual capacitance C(x,y) is Cm0. When there is a grounded conductor or finger close to the touch panel, the electric lines of force between the conductor lines X1 to Xn and Y1 to Ym may be interfered to negatively affect the values of mutual capacitances (with a value of Cm1 at contact, for example). The sensing circuits 121 measure the charges according to the change of the mutual capacitance and further generate the voltage signals Vo_1 to Vo_m.
FIG. 2 is a circuit diagram of a typical sensing circuit 121, and FIG. 3 schematically illustrates a sequence of driving signals Vin generated by the typical signal generator 110.
It is known in FIG. 3 that the signal generator 110 sequentially inputs a driving signal Vin to each of the conductor lines X1 to Xn in one direction. The driving signal Vin includes one or more pulses. The signal generator 110 uses the driving signal Vin to drive the conductor line X1 at Cycle 1, to drive the conductor line X2 at Cycle 2, and so on.
The generated pulses in the X direction pass through the coupling charges of the mutual capacitance C(1,1) to C(n,m) between the conductor lines X1 to Xn and Y1 to Ym and reach to the conductor lines Y1 to Ym in the Y direction. In this case, the sensing circuits 121 of the detection circuit 120 collect the coupling charges to thereby generate the voltage signals Vo_1 to Vo_m, so as to determine the change of the mutual capacitance according to the change of the voltage signals Vo_1 to Vo_m, and further determine whether an object approaches to the touch panel according to the change of the mutual capacitance.
In FIG. 2, C(x,y) indicates a mutual capacitance between a conductor line X and a conductor line Y. As shown in FIG. 2, when the pulses of the input driving signal Vin are inputted, the generated voltages are required to charge or discharge the integral capacitors C0 of the sensing circuits 121, wherein a sensing circuit 121 accumulates a voltage of
                  Vo              =                        Vin        ⁢                              C                          (                              x                ,                y                            )                                            C            0                                      ,which causes an additional power consumption. In addition, the common noises generated in the signal generator 110 and the conductor lines X1 to X3, such as a VCOM voltage generated in an LCD driver or a power noise generated in an LED driver, are detected and amplified by the sensing circuits 121, resulting in that the SNR (signal to noise ratio) is greatly decreased and the generated touch coordinate becomes unstable or even inaccurate.
Therefore, it is desirable for the above method for detecting a capacitive touch panel to be improved.